Methods of preparing low molecular weight carbosilanes and precursors thereof

ABSTRACT

A series of silicon compounds are provided, which are excellent precursors to small carbosilanes, such as 1,3,5-trisilapentane, 2,4,6-trisilaheptane, tris(silylmethyl)silane and tetrakis(silylmethyl)silane. A method of preparing a carbosilane involves forming a Grignard, lithium, or metallic reagent from a halomethyltrialkoxysilane, reacting the Grignard, lithium, or metallic reagent with a dihalodihydridosilane, a trihalohydridosilane, a tetrahalosilane, a dialkoxydihydridosilane, a trialkoxyhydridosilane, or a tetraalkoxysilane to yield a carbosilane precursor, and reducing the precursor to form the carbosilane.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No. 13/777,704, filed Feb. 26, 2013, which claims priority to U.S. Provisional Patent Application No. 61/622,831, filed Apr. 11, 2012, the disclosures of which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

Small, volatile carbosilanes containing only carbon, hydrogen and silicon with a carbon to silicon ratio of between 2:1 and 1:2 have been shown to be excellent precursors for the deposition of silicon carbide films in microelectronic applications. Typically, the term “small carbosilane” refers to compounds containing up to about nine backbone (silicon and/or carbon) atoms. Conventional syntheses of these low molecular weight (up to about 300 Daltons) carbosilanes include ‘in-situ’ Grignard approaches, which invariably result in the production of a vast mixture of products, including polymeric materials. It has proven very difficult to isolate pure materials from these reaction mixtures in any useful quantities. Another conventional approach to small carbosilanes involves generating silylenes in the presence of compounds containing silicon-hydrogen bonds to effect the insertion of the silylene into the Si—H bond. However, this method, upon reduction, also yields a mixture of products, including 1,1,1,3,3,3-disilapropane as a major product. Accordingly, superior methods for preparing small carbosilanes in useful quantities would be desirable.

BRIEF SUMMARY OF THE INVENTION

The invention is directed to a novel series of silicon compounds which may be used as precursors to low molecular weight compounds containing only carbon, hydrogen, and silicon. The invention is also directed to such small carbosilanes, which have favorable C:Si ratios and would be useful as precursors for the deposition of silicon carbide thin layers.

A carbosilane precursor compound according to the invention has formula (I):

wherein R¹ and R⁴ are independently selected from the group consisting of hydrogen, linear alkyl groups having one to about four carbon atoms, and linear or branched alkoxy groups having one to about eight carbon atoms, R², R³, R⁵, and R⁶ are each independently selected from the group consisting of branched alkyl groups having about three to about eight carbon atoms, and R⁷ and R⁸ are independently selected from the group consisting of hydrogen, chlorine, bromine, iodine, linear and branched alkoxy groups having one to about four carbon atoms, and linear and branched alkyl groups having one to about eight carbon atoms with hydrogen, methyl, methoxy, and ethoxy being preferred.

A method of preparing a carbosilane comprises forming a Grignard, lithium, or metallic reagent from a halomethyltrialkoxysilane, reacting the Grignard, lithium, or metallic reagent with a dihalodihydridosilane, a trihalohydridosilane, a tetrahalosilane, a dialkoxydihydridosilane, a trialkoxyhydridosilane, or a tetraalkoxysilane to yield a carbosilane precursor, and reducing the precursor to form the carbosilane.

A carbosilane compound according to the invention has formula (IX):

wherein X₁, X₂ and X₃ are each independently selected from the group consisting of hydrogen, methyl, and CH₂SiH₃.

DETAILED DESCRIPTION OF THE INVENTION

The precursor silicon compounds according to the invention are shown generally in formula (I), in which R¹ and R⁴ (representing the substituents on the terminal silicon atoms) are each independently hydrogen, a linear alkyl group containing one to about four carbon atoms, preferably one carbon atom, or a linear or branched alkoxy group having one to about eight carbon atoms, for example methoxy, ethoxy, isopropoxy, isobutoxy, tert-butoxy, and sterically hindered (branched) alkoxy groups such as cyclopentyl, cyclohexyl, and 2-ethylhexyl alkoxy groups; methoxy, ethoxy, isobutoxy, and isopropoxy groups are preferred alkoxy substituents. R², R³, R⁵, and R⁶ are each independently a branched alkyl group having about three to about eight carbon atoms, including, for example isopropyl, isobutyl, tert-butyl, cyclopentyl, cyclohexyl, and 2-ethylhexyl groups. R⁷ and R⁸, representing the substituents on the internal silicon atoms, are each independently selected from hydrogen, chlorine, bromine, iodine, linear and branched alkoxy groups having one to about eight carbon atoms (such as methoxy, ethoxy, isopropoxy, and tert-butoxy groups), and substituted and unsubstituted linear and branched alkyl groups having one to about eight carbon atoms. Preferably, R⁷ and R⁸ are each independently selected from hydrogen, methyl, methoxy, and ethoxy groups.

For the purposes of this disclosure, the term “about,” when used to refer to a number of carbon atoms, may be understood to encompass values within one whole number from the recited value. That is, a group described as having “about four carbon atoms” would encompass three, four, and five carbon atom groups.

In a preferred group of silicon compounds according to the invention, R⁷ is a substituted methyl group such as an alkyldialkoxysilylmethyl group or a trialkoxysilylmethyl group. Thus, R⁷ may be represented by the formula CH₂Si(R¹¹)(OR⁹)(OR¹²). Silicon compounds having such an R⁷ substituent are shown generally in formula (II).

In formula (II), R¹ to R⁶ are as previously defined, R⁹ and R¹² are each independently a branched alkyl group having about three to about eight carbon atoms, such as, for example, isopropyl, isobutyl, tert-butyl, cyclopentyl, cyclohexyl, and 2-ethylhexyl groups. R¹⁰ is selected from hydrogen, chlorine, bromine, iodine, alkoxy groups having about one to about eight carbon atoms (such as, but not limited to, methoxy, ethoxy, isopropoxy, tert-butoxy, cyclopentoxy, cyclohexoxy, and 2-ethylhexoxy groups), and linear and branched alkyl groups having about one to about eight carbon atoms. Preferably, R¹⁰ is selected from hydrogen, methyl, methoxy and ethoxy. Finally, R¹¹ is a linear alkyl group containing one to about four carbon atoms, preferably one carbon atom, or a linear or branched alkoxy group having one to about eight carbon atoms, such as, for example, methoxy, ethoxy, isopropoxy, isobutoxy, tert-butoxy, and longer, sterically hindered (branched) alkoxy groups, such as cyclopentyl, cyclohexyl, and 2-ethylhexyl alkoxy groups; methoxy, ethoxy, isobutoxy, and isopropoxy are preferred alkoxy substituents.

In a second preferred group of silicon compounds according to the invention, both R⁷ and R⁸ are substituted methyl groups, such that each of R⁷ and R⁸ is an alkyldialkoxysilylmethyl group or a trialkoxysilylmethyl group. Such groups may be represented by the formulas CH₂Si(R¹¹)(OR⁹)(OR¹²) or CH₂Si(R¹³)(OR¹⁴)(OR¹⁵). Groups of silicon compounds having such R⁷ and R⁸ substituents may be generally shown in formula (III).

In formula (III), R¹ to R⁶, R⁹, R¹¹ and R¹² are as previously defined, R¹⁴ and R¹⁵ are each independently a branched alkyl group having about three to about eight carbon atoms, such as, for example, isopropyl, isobutyl, tert-butyl, cyclopentyl, cyclohexyl, and 2-ethylhexyl groups. R¹³ is a linear alkyl group containing one to about four carbon atoms, preferably one carbon atom, or a linear or branched alkoxy group having one to about eight carbon atoms, such as, for example, methoxy, ethoxy, isopropoxy, isobutoxy, tert-butoxy, and longer, sterically hindered (branched) alkoxy groups, such as cyclopentyl, cyclohexyl, and 2-ethylhexyl groups alkoxy groups; methoxy, ethoxy, isobutoxy, and isopropoxy groups are preferred alkoxy substituents. Another potential route to structures of this general type would be through the reaction of bis(chloromagnesiomethyl)diisopropoxysilane or of bis(chloromagnesiomethyl)di-tert-butoxysilane, with the appropriate chlorosilanes. However, this route is currently considered to be less desirable due to the lack of availability of the requisite starting materials.

It is also within the scope of the invention to prepare carbosilane precursor compounds having longer silicon-carbon backbones than those exemplified above which contain a five atom backbone. Specifically, similar compounds having backbones containing up to about nine atoms are also within the scope of the invention.

The carbosilane precursor compounds according to the invention may be synthesized via a Grignard reagent, lithium reagent, zinc reagent, or other suitable metallic reagent, preferably a Grignard reagent, which is prepared from an appropriate halomethyltrialkoxysilane in which the trialkoxysilyl group is inert to a Grignard, lithium or metallic reagent. That is, the trialkoxysilyl group does not react with the Grignard, lithium, or metallic reagent that is formed. Methods for forming Grignard, lithium, and metallic reagents are well known in the art and need not be described. The prepared Grignard, lithium, or metallic reagent is then reacted with an appropriate dihalodihydridosilane, trihalohydridosilane, tetrahalosilane, dialkoxydihydridosilane, trialkoxyhydridosilane, or tetraalkoxysilane (preferred alkoxy groups are methoxy and ethoxy) in an appropriate solvent to yield the desired product. Appropriate reaction conditions for these reactions may be determined by routine experimentation. For example, chloromethyltriisopropoxysilane may be reacted with magnesium in an ethereal solvent to form the corresponding Grignard reagent, which is then reacted with dichlorosilane to form 1,1,1,5,5,5-hexaisopropoxy-1,3,5-trisilapentane. This compound has formula (I), in which R¹=R⁴=isopropoxy, R²=R³=R⁵=R⁶=isopropyl, and R⁷=R⁸=hydrogen.

The silicon compounds having formulas (I), (II) and (III) may be reduced at silicon to yield the corresponding carbosilane compounds containing only carbon, silicon, and hydrogen atoms. Reduction is preferred performed using lithium aluminum hydride (LiAlH₄) as a reducing agent as it provides excellent conversions. However, it is also within the scope of the invention to utilize other reducing agents which are known in the art or to be developed, including sodium aluminum hydride (NaAlH₄), Vitride® (sodium dihydrobis(2-methoxyethoxide)aluminate), and diisobutylalane. The reduction is preferably performed in a solvent such as diethyl ether, tetrahydrofuran, methyltetrahydrofuran, or di-n-butyl ether. Appropriate reaction conditions for the reduction may be determined by routine experimentation.

Silicon compounds having formulas (I), (II), and (III) thus represent valuable precursors to small, low molecular weight carbosilane compounds. Carbosilane compounds according to the invention may be generally represented by formula (IX):

In formula (IX), X₁, X₂ and X₃ are each independently selected from the group consisting of hydrogen, methyl, and CH₂SiH₃. These oligomeric, volatile carbosilanes containing only silicon, carbon, and hydrogen are potential valuable precursors to silicon carbon thin layers.

Specific carbosilane compounds according to the invention include 1,3,5-trisilapentane (formula (IV)), 2,4,6-trisilaheptane (formula (V)), tris(silylmethyl)silane (formula (VI)), tetrakis(silylmethyl)silane (formula (VII)), tris(silylmethyl)methylsilane (formula (VIII)), tris(2-silapropyl)silane (formula (X)), and tris(2-silapropyl)methylsilane (formula (XI)). Preferred carbosilane compounds are perhydridocarbosilanes wherein the Si:C ratio ranges from 3:4 to 5:3.

The invention will now be described in connection with the following, non-limiting examples.

EXAMPLE 1 Preparation of Chloromethyltriisopropoxysilane

A 22-liter flask equipped with a condenser, addition funnel, and overhead stirring was charged with isopropanol (1.19 kg; 19.80 mol), triethylamine (2.00 kg; 19.80 mol), and hexane (4.74 kg). The resulting solution was stirred at room temperature and chloromethyltrichlorosilane (1.103 kg; 6.0 mol) was added over a 4 hour period during which time the temperature ranged from 25 to 65° C. After the addition was complete, the reaction mixture was heated at 65° C. for 2 hours and cooled to 25° C. The reaction mixture was washed with water (2×1200 mL) and the organic layer dried over sodium sulfate. Distillation of the solvent and the final product gave 1.2 kg (78% yield) of chloromethyltriisopropoxysilane, by 92-4° C./11 mm Hg; d²⁵ 0.957. Gas chromatographic analysis showed the product to be >99% pure.

EXAMPLE 2 Preparation of 1,1,1,5,5,5-hexaisopropoxy-1,3,5-trisilapentane

A 5-liter flask equipped with a dry-ice condenser, magnetic stirring, and addition funnel was charged with magnesium (73.54 g; 3.03 mol) and tetrahydrofuran (300 mL). The magnesium was activated by the addition of a small amount of 1,2-dibromoethane. A pre-mix of chloropropyltriisopropoxysilane prepared in Example 1 (700.5 g; 2.75 mol) and tetrahydrofuran (1400 mL) was prepared and added to the flask at an appropriate rate to maintain the reaction temperature between 25 and 35° C. The addition time required 4.5 hours. After the addition was complete, the reaction mixture was stirred an additional 4 hours at 30° C. Titration showed the concentration to be 0.95 M. To this prepared Grignard reagent was added dichlorosilane in heptane (0.56 kg; 1.38 mol dichlorosilane) over a period of 1 hour, during which time the temperature rose to 50° C. After the addition was complete, the reaction mixture was stirred for an additional 6 hours at 40° C. The reaction mixture was filtered to remove the magnesium chloride salts and the salts were washed with hexane (2×110 mL). Solvent removal and distillation gave 325 g (50%) of the 1,1,1,5,5,5-hexaisopropoxy-1,3,5-trisilapentane, by 138° C./1.5 mm Hg; d²⁵ 0.906. Gas chromatographic analysis indicated 98% purity.

EXAMPLE 3 Preparation of 1,3,5-Trisilapentane

A 5-liter flask equipped with an addition funnel, distillation column with dry-ice condenser on top, pot thermometer, and overhead stirring all under an argon atmosphere was charged with di-n-butyl ether (1.71 kg). Lithium aluminum hydride (34.2 g; 0.90 mol) was added over a 2 hour period. After the lithium aluminum hydride had fully dissolved, 1,1,1,5,5,5-hexaisopropoxy-1,3,5-trisilapentane prepared in Example 2 (210 g; 0.45 mol) was added dropwise over a period of 2 hours while maintaining the temperature below 50° C. The reaction mixture was allowed to react an additional 30 minutes at 45° C. The product, along with some solvent, was removed by distillation directly from the reaction vessel at 50° C. and 15 torr pressure. A second distillation of the product/solvent cut provided 43 g (80.7%) of 1,3,5-trisilapentane with 98.7% GC purity.

It will be appreciated by those skilled in the art that changes could be made to the embodiments described above without departing from the broad inventive concept thereof. It is understood, therefore, that this invention is not limited to the particular embodiments disclosed, but it is intended to cover modifications within the spirit and scope of the present invention as defined by the appended claims. 

We claim:
 1. A method of preparing a carbosilane comprising: forming a Grignard, lithium, or metallic reagent from a halomethyltrialkoxysilane; reacting the Grignard, lithium, or metallic reagent with a dihalodihydridosilane, a trihalohydridosilane, a tetrahalosilane, a dialkoxydihydridosilane, a trialkoxyhydridosilane, or a tetraalkoxysilane to yield a carbosilane precursor; and reducing the precursor to form the carbosilane; wherein the carbosilane precursor is a silicon compound having formula (I):

wherein R¹ and R⁴ are independently selected from the group consisting of linear alkyl groups having one to about four carbon atoms and linear or branched alkoxy group having one to about eight carbon atoms, R², R³, R⁵, and R⁶ are each independently selected from the group consisting of branched alkyl groups having about three to about eight carbon atoms, and R⁷ and R⁸ are independently selected from the group consisting of hydrogen, chlorine, bromine, iodine, linear and branched alkoxy groups having one to about eight carbon atoms, and substituted and unsubstituted linear and branched alkyl groups having one to eight carbon atoms.
 2. The method according to claim 1, wherein the carbosilane is 1,3,5-trisilapentane.
 3. The method according to claim 1, wherein the carbosilane is 2,4,6-trisilaheptane.
 4. The method according to claim 1, wherein the carbosilane is tris(silylmethyl)silane.
 5. The method according to claim 1, wherein the carbosilane is tetrakis(silylmethyl)silane.
 6. The method according to claim 1, wherein the carbosilane is tris(silylmethyl)methylsilane. 